1. Field of the Invention
The invention relates to a liquid crystal composition suitable for use in an AM (active matrix) device and an AM device containing the composition. In particular, the invention relates to a liquid crystal composition having a negative dielectric anisotropy and a device having an IPS (in-plane switching) mode or a VA (vertical alignment) mode containing the composition.
2. Related Art
In a liquid crystal display device, classification based on an operating mode of liquid crystals includes phase change (PC), twisted nematic (TN), super twisted nematic (STN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), vertical alignment (VA), and so forth. Classification based on a driving mode includes a passive matrix (PM) and an active matrix (AM). PM is further classified into static, multiplex and so forth, and AM is classified into a thin film transistor (TFT), a metal insular metal (MIM) and so forth. TFT is further classified into amorphous silicon, polycrystal silicon and continuous grain silicon. The polycrystal silicon is classified into a high temperature type and a low temperature type according to a production process. Classification based on a light source includes a reflection type utilizing natural light, a transmission type utilizing a backlight, and a semi-transmission type utilizing both natural light and a backlight.
These devices contain a liquid crystal composition having suitable characteristics. General characteristics of the composition should be improved to obtain an AM device having good general characteristics. Table 1 below summarizes the relationship between the general characteristics of the composition and the AM device. The general characteristics of the composition will be explained further based on a commercially available AM device. The temperature range of a nematic phase relates to the temperature range, in which the device can be used. The desirable maximum temperature of a nematic phase is approximately 70° C. or more, and a desirable minimum temperature of a nematic phase is approximately −20° C. or less. The viscosity of the composition relates to the response time of the device. A short response time is desirable for displaying a moving image. Accordingly, a small viscosity of the composition is desirable. A small viscosity at a low temperature is more desirable.
TABLE 1General characteristics of liquidcrystal composition and AM deviceGeneral Characteristics ofGeneral Characteristics ofNoa Compositionan AM Device1Temperature range of a nematicUsable temperature range is widephase is wide2Viscosity is small1Response time is short3Optical anisotropy is suitableContrast ratio is large4Dielectric anisotropy is largeDriving voltage is low, electricin positive or negativepower consumption is small5Specific resistance is largeVoltage holding ratio is largeand a contrast ratio is large1A liquid crystal composition can be injected into a cell in a short time.
The optical anisotropy of the composition relates to the contrast ratio of the device. Devices having a VA mode, an IPS mode, and so forth utilize electrically controlled birefringence. In order to maximize a contrast ratio of a device having a VA mode, an IPS mode, and so forth, a product (Δn·d) of the optical anisotropy (Δn) of the composition and the cell gap (d) of the device is designed to be a constant value. Examples of the value include from approximately 0.30 μm to approximately 0.35 μm (VA mode) and from approximately 0.20 μm to approximately 0.30 μm (IPS mode). Since the cell gap (d) is generally from approximately 3 μm to approximately 6 μm, the optical anisotropy of the composition is generally from approximately 0.05 to approximately 0.11. A large dielectric anisotropy of the composition contributes to a small driving voltage of the device. Accordingly, a large dielectric anisotropy is desirable. A large specific resistance of the composition contributes to a large voltage holding ratio and a large contrast ratio of the device. Accordingly, a composition having a large specific resistance is desirable at room temperature and also at a high temperature in the initial stage. A composition having a large specific resistance is desirable at room temperature and also at a high temperature after use for a long period of time. Since an ultraviolet ray is used on producing a device, a composition having a large specific resistance is desirable at room temperature and also at a high temperature after irradiation with an ultraviolet ray.
A composition having a positive dielectric anisotropy is used in an AM device having a TN mode. On the other hand, a composition having a negative dielectric anisotropy is used in an AM device having a VA mode. A composition having a positive or negative dielectric anisotropy is used in an AM device having an IPS mode. A liquid crystal composition having a negative dielectric anisotropy is disclosed in the following patent documents: JP H10-176167 A/1998, JP 2000-96055 A/2000, JP 2001-354967 A/2001 and JP 2003-13065 A/2003.